Electromechanical valve actuators for actuating cylinder valves of an engine have several system characteristics to overcome. First, valve landing during opening and closing of the valve can create noise and wear. Therefore, valve landing control is desired to reduce contact forces and thereby decrease wear and noise. However, in some prior art actuators, the rate of change of actuator magnetic force between an armature and a core with respect to changes in the airgap length (dF/dx) can be high when the air gap is small (e.g., at landing). As such, it can be difficult to accurately control the armature and/or seat landing velocity.
Second, opening and closing time of the valve can be greater than a desired value, e.g., 3 msec. In other words, due to limited force producing capability, the transition time of some previous systems may be too slow, and therefore result in reduced engine peak power.
Third, the power consumption of an electric valve actuation (EVA) system can have an impact on vehicle fuel economy, engine peak power, and the size/cost of the electrical power supply system. Therefore, reducing power consumption of the actuator, without sacrificing performance, can be advantageous.
One approach for designing a electromechanical valve actuator of an engine with a permanent magnet is described in JP 2002130510A. Various figures show what appears to be a permanent magnet located below coils having an adjacent air gap. The objective of this reference appears to be to increase the flux density in the core poles by making the permanent magnet width (“Wm” in FIG. 4) wider than the center pole width (“Di” in FIG. 4). The air gaps 39 by the two sides of the permanent magnet appear to be introduced to limit the leakage flux. Apparently, to further increase the flux density in the core pole, the permanent magnet cross section shape is changed from flat to V-shaped in FIG. 9.
Such a configuration therefore results in the bottom part of the center pole (Wm) being wider than the top part (Di) to accommodate the permanent magnet, which is placed below the coil. The inventors herein have recognized that these two features give rise to several disadvantages.
As a first example, such a configuration can result in increased coil resistance or actuator height requirements. In other words, to provide space for the permanent magnet, either the height of the actuator is increased (to compensate the loss of the space for the coil), or the resistance of the coil is higher if the height of the core is kept constant.
As a second example, the flux enhancing effect may also be limited by actuator height. In other words, the amount of space below the coils available for the permanent magnet is limited due to packaging constraints, for example. Therefore, while some flux enhancement may be possible, it comes at a cost of (as is limited by) height restrictions.
Still another potential disadvantage with the approach of JP 2002130510A is that it can be more difficult to manufacture due to the peak of the V in FIG. 9.
Other attempts have also been made to improve the actuator performance by using a permanent magnet. For example, U.S. Pat. No. 4,779,582 describes one such actuator. However, the inventors herein have also recognized that while such an approach may produce a low dF/dx, it still produces low magnetic force due to the magnetic strength limit of the permanent magnet material. Alternatively, other approaches, such as in U.S. application Ser. No. 10/249,328, assigned to the assignee of the present invention, may increase the magnetic force, but may not reduce the dF/dx for armature and valve landing speed control.
In one example, the above disadvantages can be at least partially overcome by a valve actuator for an internal combustion engine, comprising: at least one electromagnet having a coil wound about a core; an armature fixed to an armature shaft extending axially through the core, and axially movable relative thereto; and at least one permanent magnet extending at least partially into an interior portion of the coil.
In this way, there is increased space for the permanent magnet (between the coils), and the actuator height is not required to be increased (although it can be, if desired). Additionally, since coil space is not necessarily reduced; increased coil resistance can be avoided or reduced.
In one specific example, such an approach can be used so that the area of the permanent magnet surface contacting the core is larger than the center pole area facing the armature. As a result, the flux density in the center pole surface may be significantly higher than the flux density in the permanent magnet material's surface, which is limited by the permanent magnet material property. Further, since the magnetic force is proportional to the square of the flux density, this embodiment can increase (significantly in some examples) the force, without necessarily increasing the size of the actuator. And since the permanent magnet is in the path of the flux produced by the current, in one example, the actuator can have a low dF/dx and dF/di (rate of change of force with respect to changes in current), which can be beneficial for landing speed control.
As such, various advantages can be achieved in some cases, such as decreased resistance, decreased height requirements, and increased force output, while maintaining reduced dF/dx and dF/di (which can help valve landing control).
In another example embodiment, the valve actuator can comprise a core having a wound coil located therein, said core further having at least one permanent magnet located at least partially below said coil and positioned at an angle relative to a direction of movement of an armature, with an inner part of said permanent magnet being located closer to said coil than an outer part of said permanent magnet, where said inner part of said permanent magnet is closer to a center of said core than said outer part of said permanent magnet. Further, the valve actuator can further comprise a first gap at said inner part of said permanent magnet and a second gap at said outer part of said permanent magnet.
By having such a configuration, it is also possible to obtain improved actuator force performance, while reducing coil resistance and improving valve manufacturability. Further, in some examples using gaps near selected areas of the permanent magnet, flux leakage can be reduced.
The above advantages and other advantages and features will be readily apparent from the following detailed description or from the accompanying drawings, taken alone or in combination.